Q. An operating system uses Shortest Remaining Time first (SRT) process scheduling algorithm. Consider the arrival times and execution times for the following processes:

Process Execution time Arrival time
P1 : 20 0
P2 : 25 15
P3 : 10 30
P4 : 15 45

What is the total waiting time for process P2?

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Q. Three processes A, B and C each execute a loop of 100 iterations. In each iteration of the loop, a process performs a single computation that requires tc CPU milliseconds and then initiates a single I/O operation that lasts for tio milliseconds. It is assumed that the computer where the processes execute has sufficient number of I/O devices and the OS of the computer assigns different I/O devices to each process. Also, the scheduling overhead of the OS is negligible. The processes have the following characteristics:

Process id tc tio
A : 100 ms 500 ms
B : 350 ms 500 ms
C : 200 ms 500 ms

The processes A, B, and C are started at times 0, 5 and 10 milliseconds respectively, in a pure time sharing system (round robin scheduling) that uses a time slice of 50 milliseconds. The time in milliseconds at which process C would complete its first I/O operation is ___________.

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Q. An operating system uses shortest remaining time first scheduling algorithm for pre-emptive scheduling of processes. Consider the following set of processes with their arrival times and CPU burst times (in milliseconds):

Process : Arrival Time Burst Time
P1 : 0 12
P2 : 2 4
P3 : 3 6
P4 : 8 5

The average waiting time (in milliseconds) of the processes is _________.

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Q. Consider the following set of processes, with the arrival times and the CPU-burst times given in milliseconds

Process: Arrival Time Burst Time
P1 : 0 5
P2 : 1 3
P3 : 2 3
P4 : 4 1

What is the average turnaround time for these processes with the preemptive shortest remaining processing time first (SRPT) algorithm ?

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Q. A uni-processor computer system only has two processes, both of which alternate 10ms CPU bursts with 90ms I/O bursts. Both the processes were created at nearly the same time. The I/O of both processes can proceed in parallel. Which of the following scheduling strategies will result in the least CPU utilization (over a long period of time) for this system ?

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Q. Which of the following scheduling algorithms is non-preemptive?

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Q. Consider a set of n tasks with known runtimes r1, r2, .... rn to be run on a uniprocessor machine. Which of the following processor scheduling algorithms will result in the maximum throughput?

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Q. Consider a uniprocessor system executing three tasks T1, T2 and T3, each of which is composed of an infinite sequence of jobs (or instances) which arrive periodically at intervals of 3, 7 and 20 milliseconds, respectively. The priority of each task is the inverse of its period and the available tasks are scheduled in order of priority, with the highest priority task scheduled first. Each instance of T1, T2 and T3 requires an execution time of 1, 2 and 4 milliseconds, respectively. Given that all tasks initially arrive at the beginning of the 1st milliseconds and task preemptions are allowed, the first instance of T3 completes its execution at the end of ______________ milliseconds.

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Q. The maximum number of processes that can be in Ready state for a computer system with n CPUs is

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Q. For the processes listed in the following table, which of the following scheduling schemes will give the lowest average turnaround time?

Process : Arrival Time Processing Time
A : 0 3
B : 1 6
C : 4 4
D : 6 2

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Q. Which of the following is FALSE about SJF (Shortest Job First Scheduling)?

S1: It causes minimum average waiting time
S2: It can cause starvation

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Q. Two concurrent processes P1 and P2 use four shared resources R1, R2, R3 and R4, as shown below.

P1 P2
Compute: Use R1; Use R2; Use R3; Use R4; Compute; Use R1; Use R2; Use R3;. Use R4;

Both processes are started at the same time, and each resource can be accessed by only one process at a time The following scheduling constraints exist between the access of resources by the processes:

P2 must complete use of R1 before P1 gets access to R1
P1 must complete use of R2 before P2 gets access to R2.
P2 must complete use of R3 before P1 gets access to R3.
P1 must complete use of R4 before P2 gets access to R4.

There are no other scheduling constraints between the processes. If only binary semaphores are used to enforce the above scheduling constraints, what is the minimum number of binary semaphores needed?  

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Q. We wish to schedule three processes P1, P2 and P3 on a uniprocessor system. The priorities, CPU time requirements and arrival times of the processes are as shown below.

 Process  Priority  CPU time required  Arrival time (hh:mm:ss)
 P1 10(highest)  20 sec  00:00:05
 P2  9  10 sec  00:00:03
 P3  8 (lowest)  15 sec  00:00:00

We have a choice of preemptive or non-preemptive scheduling. In preemptive scheduling, a late-arriving higher priority process can preempt a currently running process with lower priority. In non-preemptive scheduling, a late-arriving higher priority process must wait for the currently executing process to complete before it can be scheduled on the processor. What are the turnaround times (time from arrival till completion) of P2 using preemptive and non-preemptive scheduling respectively.

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Q. Consider an arbitrary set of CPU-bound processes with unequal CPU burst lengths submitted at the same time to a computer system. Which one of the following process scheduling algorithms would minimize the average waiting time in the ready queue?

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Q. Assume every process requires 3 seconds of service time in a system with single processor. If new processes are arriving at the rate of 10 processes per minute, then estimate the fraction of time CPU is busy in system?

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Q. Consider the following code fragment:

if (fork() == 0)
{ a = a + 5; printf("%d,%d\n", a, &a); }
else { a = a –5; printf("%d, %d\n", a, &a); }

Let u, v be the values printed by the parent process, and x, y be the values printed by the child process. Which one of the following is TRUE?

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Q. The atomic fetch-and-set x, y instruction unconditionally sets the memory location x to 1 and fetches the old value of x in y without allowing any intervening access to the memory location x. consider the following implementation of P and V functions on a binary semaphore .

void P (binary_semaphore *s) {
unsigned y;
unsigned *x = &(s->value);
do {
fetch-and-set x, y;
} while (y);
}

void V (binary_semaphore *s) {
S->value = 0;
}

Which one of the following is true?

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Q. Three concurrent processes X, Y, and Z execute three different code segments that access and update certain shared variables. Process X executes the P operation (i.e., wait) on semaphores a, b and c; process Y executes the P operation on semaphores b, c and d; process Z executes the P operation on semaphores c, d, and a before entering the respective code segments. After completing the execution of its code segment, each process invokes the V operation (i.e., signal) on its three semaphores. All semaphores are binary semaphores initialized to one. Which one of the following represents a deadlockfree order of invoking the P operations by the processes?

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Q. A shared variable x, initialized to zero, is operated on by four concurrent processes W, X, Y, Z as follows. Each of the processes W and X reads x from memory, increments by one, stores it to memory, and then terminates. Each of the processes Y and Z reads x from memory, decrements by two, stores it to memory, and then terminates. Each process before reading x invokes the P operation (i.e., wait) on a counting semaphore S and invokes the V operation (i.e., signal) on the semaphore S after storing x to memory. Semaphore S is initialized to two. What is the maximum possible value of x after all processes complete execution?

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Q. A process executes the code
fork();
fork();
fork();
The total number of child processes created is

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